1. Field of the Invention
This invention relates to a capacitive physical quantity sensor.
2. Description of the Prior Art
An acceleration sensor using a capacitive physical quantity sensor for detecting acceleration thereto is known. Recent acceleration sensors require a filter circuit in the signal processing circuit provided thereto for processing the signal from the sensor portion. Moreover, the filter circuit is required to have a low cutoff frequency so as to provide a low frequency range (for example, 10 to 10 KHz). This is because the acceleration signal is required to have a frequency range from zero (dc) to hundreds Hz, but a resonance frequency of the structure of the sensor exists at from hundreds to thousands Hz. This resonance frequency component should be removed. For this, a switched capacitor filter circuit (SFC circuit) is used. Moreover, the switched capacitor filter needs a relatively small area in the signal processing circuit and easily provides a low frequency range.
The SCF circuit includes analog switches comprising CMOS transistors and operational amplifiers and is miniaturized by a CMOS processing. The cutoff frequency of the SCF circuit is determined by a ratio of capacitances in the SCF circuit and a frequency of a clock signal for controlling switches in the SCF circuit.
If the signal processing circuit including such a switched capacitor filter circuit generates the clock signals for sampling and holding the voltage signal from the sensor and the clock signal for the switched capacitor filter circuit, generally, the clock signals for the sample and hold circuit should be in phase with the carrier signals for the sensor. If these signals are out of phase, the sensor may erroneously operate or the accuracy of the sensor output may decrease because of mutual clock noises.
The inventors disclosed a capacitive physical quantity sensor having a self-diagnostic function for diagnosing whether the sensor output is accurate in Japanese patent application provisional publication NO. 10-185083. FIG. 6 is a block diagram of this prior art capacitive physical quantity sensor.
This prior art sensor includes a sensor element 110 including movable electrodes 101a and 101b and fixed electrodes 102a and 102b and a detection circuit 120 for detecting acceleration on the basis of the difference capacitances between the movable electrode 101a and the fixed electrode 102a and between the movable electrode 101b and the fixed electrode 102b. The detection circuit 120 includes a C-V conversion circuit 121, a switch circuit 122, a sample and hold circuit 123, an SCF circuit 124, and a control signal generation circuit 125 for generating clock signals. The C-V conversion circuit 121 converts variation in the difference capacitance of the movable electrodes 101a and 101b and the fixed electrodes 102a and 102b. Next, the sample and hold circuit 123 samples and holds the sensor output. The SCF circuit 124 filters the sampled sensor output.
FIGS. 7A to 7G are timing charts of signals for self-diagnosis in the prior art sensor.
In FIGS. 7A to 7G, the carrier signals PW1 and PW2 supplied to the fixed electrodes 102a and 102b,a switch signal ST for switching the reference voltage, a signal S1 for switch 121c, and the circuit clock S2 for the sample and hold circuit 123 and SCF circuit 124 are changed in the period between the measuring (M) mode and the self-diagnostic (SD) mode. That is, the circuit clock S2 for the switched capacitor filter circuit 124 is commonly used in sample and hold circuit 123. Accordingly, the circuit clock S2 for the switched capacitor filter circuit 124 is varied between the measuring and self-diagnostics modes.
The aim of the present invention is to provide a superior capacitive physical quantity sensor.
According to the present invention, a first aspect of the present invention provides a capacitive physical quantity sensor comprising: first and second variable capacitors, each including a movable electrode and a fixed electrode facing each other, capacitances of said first and second variable capacitors varying in accordance with a physical quantity on said movable electrode; signal generation means for periodically supplying carrier signals to said fixed electrodes to measure variation in differential capacitance of said first and second variable capacitors at a first period in a measuring mode and at a second period in a self-diagnostic modes and generating a displacement signal for displacing said movable electrode at said second period in said self-diagnostic mode; and a signal processing circuit including: a C-V conversion circuit for converting a charge signal indicative of said differential capacitance into a voltage signal; and a switched capacitor filter circuit for filtering said voltage signal to output a filtered voltage signal in response to a filter circuit clock signal, wherein said signal generation means further generates said filter circuit clock signal in said measuring and self-diagnostic modes at the same period, and said first period is different from said second period.
According to the present invention, a second aspect of the present invention provides a capacitive physical quantity sensor based on the first aspect, wherein said signal processing circuit further comprises a sample and hold circuit for sampling and holding said voltage signal in response to a sample and holding clock signal which is different from said filter circuit clock signal in period.
According to the present invention, a third aspect of the present invention provides a capacitive physical quantity sensor based on the first aspect, wherein said signal processing circuit further comprises a sample hold circuit for sampling and holding said voltage signal in response to a sample and holding clock signal of which period is different between said measuring and self-diagnostic modes.
According to the present invention, a fourth aspect of the present invention provides a capacitive physical quantity sensor based on the second aspect, wherein said signal generation means further generates said sampling and holding clock signal and said filter circuit clock signal and further includes synchronizing means for synchronously outputting said sampling and holding clock signal and said filter circuit clock signal.
According to the present invention, a fifth aspect of the present invention provides a capacitive physical quantity sensor based on the third aspect, wherein said signal generation means further generates said sampling and holding clock signal and said filter circuit clock signal and further includes synchronizing means for synchronously outputting said sampling and holding clock signal and said filter circuit clock signal.
According to the present invention, a sixth aspect of the present invention provides a capacitive physical quantity sensor based on the fourth aspect, wherein said signal generation means includes a programmable counter circuit for generating said carrier signals at said first period and said second period in said measuring and self-diagnostic modes, respectively.
According to the present invention, a seventh aspect of the present invention provides a capacitive physical quantity sensor based on the fifth aspect, wherein said signal generation means includes a programmable counter circuit for generating said carrier signals at said first period and said second period in said measuring and self-diagnostic modes, respectively.
According to the present invention, an eighth aspect of the present invention provides a capacitive physical quantity sensor based on the first aspect, wherein said signal generation means includes an oscillator for generating a reference clock signal; a counter circuit responsive to said reference clock signal for generating said filter circuit clock signal with the same dividing ratio in said measuring and self-diagnostic modes; a programmable counter circuit responsive to the reference clock signal for generating said sample and hold clock signal, and said clock signal in said measuring mode and self-diagnostic mode with different dividing ratios, respectively.